Flip-chip semiconductor devices are well known in the electronics industry, in which a typically packageless semiconductor device, such as an integrated circuit, etc., is mounted to a substrate, such as a printed circuit board (PCB) or printed wiring board (PWB), in a die-down or face-down position, that is, with the electrical interconnects facing downward toward the substrate. That is, an active surface of a semiconductor die is mounted facing the substrate, typically by bonding a plurality of conductive bumps, for example, solder bumps on the interconnect pads of the active surface, to corresponding electrical terminals on the mounting surface of the substrate. The electrical connections between the semiconductor die and the substrate are thereby confined to an area not exceeding the size of the die, i.e., its so-called footprint size.
In order to achieve a structurally robust and environmentally resistant attachment of the flip-chip device to the substrate, it is known to use an undercoat adhesive, typically a thermosetting epoxy adhesive, sandwiched between the active face of the flip-chip device and the underlying mounting surface of the substrate. The epoxy is placed in an uncured state around the periphery of the flip-chip device after electrical connection has been established. The epoxy wicks by capillary action to at least partially fill the cavity between the flip-chip device and the substrate, following which it is cured in an oven or the like. Substantial assembly complexity is incurred in carrying out this undercoating operation. In addition, the amount of low cost, low expansion filler, such as SiO.sub.2, which can be added to the thermosetting epoxy is limited by the need to maintain a sufficiently flowable rheology.
Most flip-chip devices use solder to connect the flip-chip to the substrate. Soldering has the disadvantage of requiring post assembly heating to cause the solder to melt and flow. This heating may damage substrates that are not capable of withstanding high temperature exposure. These substrates melt, soften or degrade at between 100.degree. C. and 350.degree. C. Additionally, soldering causes the flip-chip devices to be fixed to multiple points on the substrate. These points are subject to stress when the flip-chip undergoes thermal expansion and contraction. The flip-chip device is usually made of silicon and has a thermal expansion coefficient of .about.3 ppm/.degree. C. When using thermoplastic substrates, the thermal expansion coefficient is between 15 and 150 ppm/.degree. C. Repeated cycling from hot to cold causes the substrate and flip-chip device to expand and contract at very different rates and causes stress and fatigue on the solder connection resulting in interconnection crack failure and electrical failure. Also, solder often contains lead and other materials such as fluxes. Regulations preclude or limit lead and fluxes must be removed prior to applying adhesives. Residual traces of flux may damage or degrade the connection.
Several U.S. patents show adhesively bonding the flip-chip to the substrate. Most have used conductive particles or conductive adhesive between the flip-chip device and substrate. U.S. Pat. Nos. 5,578,527 and 5,611,884 are examples which teach placing conductive particles in the adhesive. In the case of the U.S. Pat. No. 5,611,884 patent, the conductive particles must be precisely aligned with the flip chip and substrate and are affixed using a conductive adhesive. Care must be taken to avoid electrically interconnecting adjacent interconnects. In the case of the U.S. Pat. No. 5,578,527 patent, a film having uniformly dispersed conductive particles is applied between the flip-chip device and substrate. The particles are trapped between the flip-chip and substrate. The difficulty arising in placing the particles in close enough spacing to be trapped between the flip chip and substrate but not too close so as to electrically connect adjacent interconnects. Use of conductive particles and conductive adhesives is expensive and complex. Furthermore, the use of conductive particles limits the electrical joint to very low current carrying and low temperature operation. The particle to particle contact is essentially a point contact with only a very limited surface contact that cannot support high currents. Electrical connection through the particles is limited to a narrow temperature range because thermal expansion of the adhesive tends to cause the particles to separate.
Another disadvantage of current flip-chip undercoat technology is the inability to repair or replace the flip-chip device following adhesive cure. Flip-chips with soldered interconnects must be heated to the solder melting point. This heating may damage the components or the substrate. The flip-chips with thermoset adhesives do not melt and do not easily allow replacement of a defective flip-chip.
It is an object of the present invention to provide solderless flip-chip assemblies that have good performance characteristics. It is a particular object of at least certain preferred embodiments to provide repairable flip-chip assemblies and methods for the reversible assembly of flip-chip devices to electrical interconnecting substrates. These and additional objects and features will be apparent from the following disclosure of the invention and detailed description of certain preferred embodiments.